Optimized flash memory device for miniaturized devices

ABSTRACT

An implantable medical device have an associated memory device is disclosed. The implantable medical device utilizes techniques for optimizing one or more embedded operations of the memory device, such operations including programming, reading or erasing data. The techniques for optimizing the embedded operations include controlling the operations as a function of an energy source of the implantable medical device.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/059,544 (now allowed), filed Oct. 22, 2013 entitled “OPTIMIZED FLASHMEMORY DEVICE FOR MINIATURIZED DEICES”, which is herein incorporated byreference in its entirety and also claims priority to provisional U.S.Application No. 61/876,431, filed Sep. 11, 2013, which is incorporatedherein by reference in its entirety.

FIELD

The present disclosure pertains to implantable medical devices and moreparticularly to implantable medical devices having memory devices withoptimized embedded operations.

BACKGROUND

There are a variety of medical devices that sense data, providediagnostic information, and/or deliver therapy. When such a device isimplanted (in whole or in part), it is referred to as an implantablemedical device (IMD). Examples of IMDs are implantable loop recorders,implantable pacemakers, and implantable cardioverter-defibrillators,which are electronic medical devices that monitor the electricalactivity of the heart and/or provide electrical stimulation to one ormore of the heart chambers, when necessary.

As IMD technology advances in an attempt to address a myriad of lifesustaining/enhancing needs, issues such as IMD battery longevity, IMDmass, and increased functionality remain key considerations in the IMDdesign process. A conventional approach to providing power within an IMDinvolves the use of a self-contained battery, not unlike a commonbattery which is commercially available to the consumer. Such aself-contained battery includes active electrochemical cell componentshoused in a battery can. Battery housing connectors or contacts areprovided for establishing electrical connections to circuitry disposedwithin the IMD.

The functions attributable to the IMD may be impacted by the depletionof the battery energy. For example, each IMD generally includes aprocessor that executes “operation instructions” or applies “operationcode” to carry out the various operational functions of the IMD. Typicaloperation instructions are stored in one or more non-volatile memorymodules in the IMD. In addition, data sensed by the IMD is stored in theone or more volatile memory modules. Non-volatile memory includes, forexample, flash memory, read-only memory (ROM), electrically erasableprogrammable read-only memory (EEPROM), and a non-volatile random-accessmemory (NVRAM).

Much attention has been placed on optimizing the functions of the IMD tooptimize consumption of the power source of an implantable medicaldevice. The battery consumption is always a concern when designingimplantable medical devices, but this concern is increased for smallform factor devices that can only accommodate a small battery canister.There remains a need for increased optimization of the operationalfunctions of the IMD pertaining to the storage operations in thevolatile memory modules.

SUMMARY

Generally, the disclosure is directed to the design of one or morememory devices within an implantable medical device (IMD). In accordancewith aspects of the present invention, techniques are disclosed forimplementing a memory device with a programming delay enables thedownsizing of the battery size and capacity of the IMD.

In one aspect, an implantable medical device including a battery, aprocessing unit, and a memory device is disclosed. The memory deviceincludes a memory sector including a plurality of memory elements, and astate machine that is configured to control a timing associated with aprogramming operation of the memory sector as a function of a parameterof the battery.

The programming operation may be controlled by varying a timing ofwriting of data to a given memory element of the memory sector as afunction of a programming delay duration following a preceding writingof data to a first memory element.

In another aspect, a method of programming a memory device of animplantable medical device is disclosed. The method includes the tasksof computing a parameter of a battery of the implantable medical device,erasing a memory sector of the memory device, performing an iterativeprogramming of a plurality of elements in the memory sector, andapplying a programming delay prior to programming at least one of theplurality of elements, wherein the programming delay is a function ofthe computed parameter.

In an embodiment, a value of the programming delay is defined as afunction of the parameter of the battery.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages of the disclosure will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent disclosure and therefore do not limit the scope of thedisclosure. The drawings (not to scale) are intended for use inconjunction with the explanations in the following detailed description,wherein similar elements are designated by identical reference numerals.Moreover, the specific location of the various features is merelyexemplary unless noted otherwise.

FIG. 1 is a conceptual diagram of a therapy system illustrating afrontal view of a patient in whom an implantable medical device may beimplanted;

FIG. 2 illustrates another embodiment of a conceptual diagram of atherapy system having an implantable medical device implanted in apatient;

FIG. 3 is a block diagram of an embodiment of operational circuitry thatmay be utilized within the implantable medical devices of FIGS. 1 and 2in accordance with embodiments of the present disclosure;

FIG. 4 provides a block diagram of one embodiment of a memory device ofan implantable medical device of the present disclosure;

FIG. 5 depicts a graph representing an embodiment of the state machineof the memory devices of the present disclosure;

FIG. 6A shows a plot of the sector program time verses the cumulativecurrent drain without implementing a program delay; and

FIG. 6B shows a plot of the sector program time verses the cumulativecurrent drain with a program delay that is selected based upon thecapacity of the battery.

DETAILED DESCRIPTION

The present invention relates to configuration of flash memory devicesto optimize power consumption of battery-powered devices such asimplantable medical devices. As is generally known in the art, flashmemory devices are a class of non-volatile memory devices. A flashmemory device offers non-volatile storage of data, and also convenientlyallows the data to be programmed (written) into the memory and erasedfrom the memory multiple times, thus allowing a multitude of flexibleapplications. Such flash memory devices provide electrical erasing and asmall cell size. In a flash memory device, a plurality of one-transistorcore cells may be formed on a semiconductor substrate in which each cellis comprised of a P-type conductivity substrate, an N-type conductivitysource region formed integrally with the substrate, and an N-typeconductivity drain region also formed integrally within the substrate. Afloating gate is separated from the substrate by a thin dielectriclayer. A second dielectric layer separates a control gate from thefloating gate. A P-type channel region in the substrate separates thesource and drain regions.

One type of architecture used for flash memories is typically referredto as a NOR flash memory architecture which is an array of flash cellsthat are divided into a plurality of sectors. Further, the memory cellswithin each sector are assembled from memory elements that may compriseof multiple memory segments. A typical segment may include a few cellsthat make up a machine word (or byte) and each of the segments can bewritten or read independently.

In a typical NOR flash, the source region of each cell transistor withineach sector is tied to a common node. Therefore, all of the cells withina particular sector can be erased simultaneously or erasure may beperformed on a sector-by-sector basis. In order to program the flashcell, the drain region and the control gate are raised to predeterminedpotentials above the potential applied to the source region. Forexample, the drain region has applied thereto a voltage V_(D) ofapproximately +5.5 volts with the control gate V_(G) having a voltage ofapproximately +9 volts applied thereto. These voltages produce “hotelectrons” which are accelerated across the thin dielectric layer andonto the floating gate. The hot electron injection results in anincrease of the floating gate threshold by approximately two to fourvolts. To avoid unnecessarily obfuscating the novel aspects of thepresent invention, known details of operations of flash memory deviceswill not be described herein. The reader is referred to the descriptionof operations in U.S. Pat. No. 8,462,564, “Flash memory programmingpower reduction” to Yonggang et al., incorporated herein by reference inits entirety.

For erasing the data in a flash cell, a positive potential (e.g., +5volts) is applied to the source region. The control gate is applied witha negative potential (e.g., −8 volts), and the drain region is allowedto float. A strong electric field develops between the floating gate andthe source region, and a negative charge is extracted from the floatinggate to the source region by way of Fowler-Nordheim tunneling.

A flash memory device performs all its embedded operations, includingthe programming or erasing, based on the generation and control of manydesign parameters, such as analog signals, maximum values, timings, etc.The embedded operations in a flash memory device are complex, especiallywith regard to the generation and control of parameters and require veryfine control. These parameters have to be properly defined and, fromtime to time, updated in the various phases of the operation in order tohave perfect control of the full operation. The values of theseparameters may depend on information stored in a series of registersthat are associated with these parameters.

The present invention describes an architecture which introduces a highlevel of configurability in the flash memory devices and associateddesign parameters of a flash memory device. In the present invention, aflash memory device is described having operations that are handled by aprocess that is implemented by program instructions that may be storedin a read-only memory (ROM) and executed by a microcontroller. Forexample, the microcontroller may issue instructions to initiate aprogram or erase operation to implement the corresponding program orerase flash memory operation. The instruction is interpreted by theflash memory device, and specific signals are then generated to controlthe flash memory device in order to perform the requested flash memoryoperation. All of the needed configurations for each parameter in eachoperation phase are obtained by loading the corresponding information(values) into the associated registers.

As used in this disclosure, the term “program” refers to the operationof writing data to a location of memory. The words program and writewill be used interchangeably in this disclosure.

As used in this disclosure, the term “conditional delay” refers to acontrollable duration of time that may be selected to delay successiveoperations of two memory elements. Such operations may include aprogramming operation or an erasing operation. For simplicity, thisdisclosure will refer to a programming delay, although it should beunderstood that the concepts can be applied to an erasing delay withminimal modification that is within the scope of those skilled in theart with the benefit of this disclosure.

To more particularly describe the features of the present invention,please refer to FIGS. 1 through 6A-B in conjunction with the discussionbelow.

FIG. 1 is a conceptual diagram of a therapy system illustrating afrontal view of a patient 2 in whom an IMD 10A may be implantedsubcutaneously with a typical implant location referenced thereon (otherimplant locations may be utilized). The IMD 10A senses cardiacelectrical activation signals via electrodes (not shown in FIG. 1) fromheart 4. A communication link 12 allows 2-way telemetry communicationbetween IMD 10A and an external device (typically a programmer) 14.Programmer 14 and communication link 12 suitable for use in the practiceof the present invention are known. Known programmers typicallycommunicate with an implanted device such as IMD 10A via abi-directional radio-frequency telemetry link, so that the programmer 14can transmit control commands and operational parameter values to bereceived by the IMD 10A, and so that the IMD 10A can communicatecaptured and stored diagnostic and operational data to the programmer14. Programmers 14 believed to be suitable for the purposes ofpracticing the present invention include the Models 9790 and CareLink®programmers, commercially available from Medtronic, Inc., Minneapolis,Minn. Various telemetry systems for providing the necessarycommunications channels between programmer l4 and an IMD 10A have beendeveloped and are well known in the art and are discussed, for example,in the following U.S. patents: U.S. Pat. No. 5,127,404 to Wyborny et al.entitled “Telemetry Format for Implanted Medical Device”; U.S. Pat. No.4,374,382 to Markowitz entitled “Marker Channel Telemetry System for aMedical Device”; and U.S. Pat. No. 4,556,063 to Thompson et al. entitled“Telemetry System for a Medical Device”.

FIG. 2 illustrates another embodiment of a conceptual diagram of atherapy system having an IMD 10B that may be used to monitor one or morephysiological parameters of patient 2 and/or to provide therapy to heart4 of patient 2. IMD 10B may be an implantable leadless pacemaker thatprovides stimulation therapy signals to heart 4 via one or moreelectrodes (not shown in FIG. 2) on its outer housing. In alternativeembodiments, the IMD 10B may include one or more medical electricalleads (not shown) having electrodes for sensing and delivery electricalstimulation. Additionally or alternatively, IMD 10B may sense electricalsignals attendant to the depolarization and repolarization of heart 4via electrodes on its outer housing. In some examples, IMD 10B providespacing pulses to heart 4 based on the electrical signals sensed withinheart 4.

In the example of FIG. 2, IMD 10B is positioned wholly within heart 4with one end proximate to the apex of right ventricle to provide rightventricular (RV) pacing. Although IMD 10B is shown within heart 4 andproximate to the apex of a right ventricle in the example of FIG. 2, IMD10B may be positioned at any other location outside or within heart 4.For example, IMD 10B may be configured for implantation on an exteriorwall of heart 4, or within a blood vessel in an alternativeimplementation. Depending in the location of implant, IMD 10B mayinclude other stimulation functionalities. For example, IMD 10B mayprovide atrioventricular nodal stimulation, fat pad stimulation, vagalstimulation, or other types of neurostimulation. In other examples, IMD10B may be a monitor that senses one or more parameters of heart 4 andmay not provide any stimulation functionality.

Programmer 14 is depicted as being in communication with IMD 10B. Asdescribed above with respect to IMD 10A of FIG. 1, programmer 14 mayalso be used to communicate with IMD 10B. A user may use programmer 14to retrieve information from IMD 10B regarding the performance of IMD10B and to interact with programmer 14 to program, e.g., selectparameters for, any additional therapies provided by IMD 10B. Theprogrammer 14 also facilitates user interaction remotely via a networkedcomputing device.

IMD 10A and 10B (collectively “IMD 10”) may each include an internalpower source such as a battery that supplies power to the components ofthe IMD 10 for achieving various functions of the IMD 10. With theadvancements in the miniaturization of the implantable medical devices,such as IMD 10, the power source is also preferred to have a smallerfootprint and hence decreased capacity for storage of energy. Therefore,the inventors of the present invention have observed that there is aneed to optimize current consumption during the operation of thecomponents of IMD 10. In particular, the inventors have disclosed hereintechniques for optimizing operations of components associated with highcurrent events, such as the embedded operations of the memory devicesincluding program and erase operations. In the present device, thememory devices include a flash memory device (not shown in FIGS. 1 and2) that stores sensed data and/or data pertaining to functionality ofIMD 10 such as operating parameters, commands, and instructions.

FIG. 3 is a block diagram of an embodiment of operational circuit 18that may be utilized within miniaturized devices such as IMD 10 inaccordance with the present disclosure. It is common to providecircuitry in the IMD 10 for achieving a variety of functionality tocontrol therapy delivery and sensing functions. Portions of operationalcircuit 18 may be of conventional design such as disclosed in U.S. Pat.No. 5,052,388 issued to Sivula et al. For example, operational circuit18 may include sense amplifier circuitry 22, an optional electricalstimulating generator 24, an input/output (I/O) Bus 26, and arandom-access and/or read-only memory (RAM/ROM) device 28.

Operational circuit 18 includes a clock unit 30. The clock unit 30provides clock signals of one (or a plurality of differing) frequenciesas desired for operation of the various segments of operational circuit18. In one example, the clock unit 30 may include a high accuracyoscillator such as a crystal oscillator that provides signals forcalibrating various low-power clocks. The single calibrating source,high accuracy oscillator, coupled with several low-power clocks may beprovided to reduce the current consumption of the operation circuitry18.

The present invention provides additional techniques for optimizingpower consumption of implantable medical devices, such as IMD 10.

A central processing unit (CPU) 32 is also provided for executinginstructions stored in memory, including memory device 28, to cause IMD10 to perform various functions attributed to the IMD 10. Such functionsinclude controlling sense amplifier circuitry 22 to monitor/sensesignals associated with the electrical activity of the heart 4, and/orcausing stimulating therapy generator 24 to deliver stimulation therapyin the form of pacing pulses or defibrillation to heart 4 according tooperational parameters or programs, which may also be stored in memorydevice 28.

CPU 32 is provided to control operations of the memory device 28 and tosend addresses for manipulation of data in memory device 28. Theoperations of the memory device 28 may include such things as erasingand programming data. In one embodiment, the CPU 32 may determine alocation within the memory device 28 at which data is to be stored(programed). The CPU 32 may also determine a location within the memorydevice 28 at which data is to be retrieved (read). As will be describedin more detail below, the CPU 32 sends appropriate instructions/commandsto initiate an operation by memory device 28 based on a desiredfunctionality of IMD 10. For example, the functionality may involveacquiring physiological signals from patient 2. Without intending to belimiting, the IMD 10 will sense the physiological signals through anelectrode/electrode pair, transmit the sensed signals to the CPU 32 forprocessing, send the results of the processing to the memory device 28for storage and/or send the raw sensed signals to the memory device 28for storage. CPU 32 can be any of a variety of suitable controllerdevices, including microprocessors, application specific integratedcircuits (ASICs), or other circuits or controllers.

A communications unit such as telemetry system 34 may be provided toallow the device to communicate with external devices such as programmer14 and other devices via antenna 36 along communication channel 12.Examples of communication techniques may include, for example, lowfrequency or radiofrequency (RF) telemetry, but other techniques arealso contemplated. In some embodiments, the IMD 10 receives telemeteredsignals that may include data, instructions, and/or commands that mayalter one or more functions or operations of the IMD 10. One suchfunction pertains to the storage and retrieval of information from thememory device 28, as will be described in more detail below.

In alternative embodiments, additional exemplary electrical componentsthat may be included in the operational circuit 18 are further describedin the circuit of the device(s) in U.S. Pat. No. 5,987,352, “MinimallyInvasive Implantable Device for Monitoring Physiologic Events” to Kleinet al., incorporated herein by reference in its entirety.

The operational circuit 18 may be powered by a finite-capacity powersource such as battery 38. The battery 38 may be rechargeable, but inmost implantable medical devices, such as IMD 10, may benon-rechargeable. As a result, one design constraint that was consideredby the inventors of the present invention is ensuring that the functionsof the IMD 10 are optimized to facilitate the fulfillment of the powersupply demands from each component of the operational circuit 18. Inparticular, the inventors have addressed challenges associated with thereduced footprint, and hence storage capacity, of the battery 38 inmeeting the peak power supply demands of high current operations. Thosehigh current operations include the erase/program operations associatedwith the memory device 28.

FIG. 4 provides a block diagram of one embodiment of memory device 28.The memory device 28 associated with IMD 10 may refer to one or moretypes of memory associated with implantable medical devices, includingflash memory, random-access memory (RAM), read-only memory (ROM),electrically erasable programmable read-only memory (EEPROM), and anon-volatile random-access memory (NVRAM). For purposes of the presentinvention, the memory device 28 includes at least a flash memory array40. In an embodiment, the flash memory array 40 may be include memorycells of a static RAM (SRAM) as is described in commonly owned andassigned U.S. patent application Ser. No. 13/663,099, “MEMORY ARRAY WITHFLASH AND RANDOM ACCESS MEMORY AND METHOD THEREFOR” to Walsh et al. Thememory device 28 is functionally coupled to various components of theoperational circuit 18 through a system bus 48.

In one implementation, the memory device 28 includes a state machine 42for executing commands/instructions, (pre-determined) simple commands(or command sequences), from the CPU 32 based on received data. Examplesimple commands facilitate operations including, read, write, copy,erase, and so forth, of data residing in the flash memory array 40and/or data received for storage in the memory device 28. In alternativeembodiments, the functions attributed to state machine 42 may beperformed directly by CPU 32 or a microcontroller, co-processor,microprocessor (or any combination of a microcontroller, co-processor,microprocessor, or state machine) that are within or located external tothe memory device 28. In one implementation, a portion of the memorydevice 28 may be utilized for storage of data and program instructions(i.e., code) used to implement operations of the state machine 42 formanipulation or modification of the flash memory array 40.

Briefly, when the CPU 32 wants to access the flash memory array 40, itmay send a request containing the targeted data address to the statemachine 42. The state machine 42 then sends the proper commands to thesegment, element, or sector of flash memory array 40 to perform therequested memory access operation. The commands may cause erasure of thesector and then rewriting of the new data into the sector. During a datawrite, the full flash memory may be erased and rewritten, a memorysector may be erased and rewritten, a memory element may be erased andrewritten, or a memory segment may be erased and rewritten—dependingupon the flash architecture. In the example embodiments below, a sectorerase and write will be used to illustrate the embodiment.

Charge pumps 44 are included in the memory device 28 and are regulatedto provide voltage signals to the flash memory array 40. These voltagesignals control particular voltages needed during embedded operations ofthe flash memory array 40. For example, the cell gate voltage during aprogram operation, the cell source or bulk voltage during an eraseoperation, and the cell gate voltage during a verify operation areprovided by the charge pumps 44. Control signals for regulating thevoltage signals from the charge pumps 44 may be generated by the statemachine 42 or a processor located externally to the memory device 28.

Memory device 28 further includes one or more register bank(s) 46(collectively “register 46”) that are used to control parameters of theembedded operations of flash memory array 40. The operations arecontrolled according to configuration values that are loaded into theregister 46 and which are appropriate to a particular current phase ofthe state machine 42 for a given flash memory operation. Each register46 is therefore associated with a particular design parameter that isused in an ongoing operation of the memory device 28. For example, aregister can be associated with a parameter that is an analog devicesignal (a voltage or current), or registers can be used to configurevarious parameters associated with the read, write (program), or eraseoperations of the flash memory array 40. Each design parameter may havetwo or more values that are stored within a corresponding two or more ofthe registers in register 46. The values of the parameters may bepredetermined and pre-coded into the registers, or dynamically providedby the CPU 32 as a function of one or more operations of the CPU 32. Inuse, the state machine 42 accesses and loads the configuration valuesassociated with one or more of the parameters, which are stored in theregister 46, to implement operations of the state machine 42 formanipulation or modification of data in the flash memory array 40.

In accordance with the present disclosure, the register 46 may beimplemented to store multiple configuration values associated with aconditional delay parameter for implementing the state machine 42. Themultiple configuration values may have different values such that eachof the values is associated with a different timing when implemented bythe state machine 42. For example, a plurality of the timing values maybe stored in register 46, each of the values being associated with aduration parameter. The multiple values of the duration parameter may bedifferent and are associated with a conditional delay of an operation ofthe flash memory array 40. For example, the conditional delay may be aprogramming delay of a programming operation of the flash memory array40. Although not shown in FIG. 4, a number of multiplexers may beprovided in the memory device 28. At the inputs of each multiplexer arethe individual registers associated with a given parameter, whichcontain the different configuration values pertinent to the particularparameter. The multiplexers may be used to select one of the inputs tothe multiplexer to pass a configuration value through to the output ofthe multiplexer

In one implementation, the state machine 42 receives instructions fromCPU 32 for performing an operation such as accessing a location of theflash memory array 40 to store or retrieve data, for example. Suchinstructions to the state machine 42 may include the address or locationwithin the memory array 40 to be accessed and the memory deviceoperation to be performed. Receipt of the instructions from CPU 32initializes the launch of the state machine 42 to perform the requiredoperation. Consequently, the state machine 42 issues commands to controlthe charge pump 44 and to load values associated with the controlparameters for fulfilling the required operation.

In accordance with the present invention, one such control parameter isthe programming delay associated with programming data into the flashmemory array 40. An appropriate programming delay value is loaded intothe state machine 42 from the register 46 to enable the operation ofprogramming of data.

The selection of a programming delay value during a given programmingoperation may be based on predetermined criteria. The criteria forselection of the programming delay value may include parametersassociated with the battery 38. For example, the parameters may includea remaining energy stored in the battery (battery capacity), aneffective impedance of the battery, a peak-demand current output, or anyother desired current battery performance indicator. In accordance withthe present invention, each programming delay value is associated with acalculated parameter of the battery. The use of the remaining batteryenergy as the control parameter may be of particular interest withrespect to high current operations—during which, it is desirable toensure that the battery can supply predetermined levels of peak currentto the circuit. In the example of the parameter being the batterycapacity, one or more programming delay values may be provided to modifythe programming operation based on the remaining energy stored by thebattery. The programming delay values are defined to provide sufficienttime for the battery 38 (and other associated power supply componentssuch as capacitors) to recover after each programming operation as willbe discussed with reference to FIG. 5.

In an exemplary implementation, the individual ones of register 46 mayeach be loaded with a different programming delay value. For example,the values may range from 1.0 microsecond (μs) to 400 μs, although itshould be understood that other values outside of this range may beutilized. In any event, the selection of one of those values (within oroutside the range) may be based on a (delay) duration that enables thebattery 38 to recover sufficiently to meet the current demands (peak orotherwise) for all concurrently-running operations of IMD 10 that are tobe powered by the battery 38 at a given time.

In the description of the aforementioned embodiment of FIG. 4, theprogramming delay value has been described as being calculated with eachrequested memory access operation (programming, erasing etc.). However,it should be understood that in various other implementations, theprogramming delay value may be calculated at other intervals. Forexample, alternative embodiments may calculate the programming delayvalue at the beginning of the battery life with the value being usedthroughout the entire device operation, or the programming delay valuemay be calculated (once or multiple value) in response to the remainingbattery energy reaching a predetermined value, or any other desiredinterval during the operating life of the battery.

Turning to FIG. 5, a graph representing an embodiment of the statemachine 42 of FIG. 4 is illustrated. The operation depicted in the statemachine 42 is that of programming data into a memory location of theflash memory array 40 as a function of the values for various controlparameters supplied by register 46 (including a programming delay) andexternal signals from the CPU 32. Nevertheless, this depiction is notintended to be limiting of the operations performed by the state machine42. Rather, it should be understood that the state machine 42 maysuitably be configured to control other operations of the flash memoryarray 40 such as reading or merely erasing data. The state machine 42may be implemented in the form of a logic network of any known type. Inthe illustrated embodiment, the state machine includes four statesindicated as S0, S1, S2, and IDLE (i.e., deactivated). Each of thestates generally corresponds to a given operation of the memory device28. The state machine 42 is configured to change states as a function ofan internal timer and/or as a function of external events suitablysynchronized.

In the illustrated embodiment, the state machine 42 is initialized by anexternal event (e.g., command or signal) received from CPU 32. Forsimplicity, the external event will be referred to herein as a PROGRAMcommand. The PROGRAM command instructs state machine 42 to store data inthe flash memory array 40.

In response to receipt of the PROGRAM command from the CPU 32, the statemachine 42 extracts the parameters stored in register 46 that areassociated with the PROGRAM command. Initialization of the state machineby the PROGRAM command causes passage from the IDLE state 50 to stateS0—erase operation 52. In accordance with an embodiment of the presentinvention, a sector of the flash memory array 40 comprises of 512 bytes.However, it should be appreciated that the sector may be configuredhaving any other size and number of bytes that may be smaller, e.g., 256bytes or larger, e.g., 4 Kbytes. In the implementation of a NOR flashmemory array, the entire sector is erased before data is written.

At S2, the state machine implements a program delay 54 between thewriting of data to each memory element. The memory element refers to aconstituent unit of the memory sectors of flash memory array 40. Inother words, each memory sector may comprise two or more memory elementsthat can be written to individually. In one implementation, a sector ofthe flash memory array 40 comprises 512 bytes and each sector comprisesof 128 memory elements (i.e., each element is 4 bytes long). Further, itshould be understood that the memory elements may comprise of evensmaller units (memory segments) and so forth. Nevertheless, to avoidunnecessarily obfuscating the invention, the smallest divisible unitdescribed herein will be the memory element.

At S3, a program operation 56 of data to the flash memory array 40 isperformed by sequentially writing to each memory element in the sector.The programming 56 is performed in several iterations, with each of theiterations comprising writing the data to a first of the memory elementsfollowed by a program delay 52 and then writing data to a second of thememory elements followed by another program delay 52 and so forth. Inother words, the state machine 42 loops between states S2 and S3 duringthe programming operation 56 until completion of the writing to thememory elements in a given sector. As such, if the sector includes twomemory elements, then only one programming delay will be introducedbetween the programming of the first and second elements, or if thesector includes three memory elements, then two programming delays willbe injected and so forth. Thus, a programming loop is defined by thestates S2 and S3.

As previously described, the value of the program delay 54 is determinedbased on a parameter of the battery 38. In the non-limiting example ofthe parameter being the capacity of the battery 38, the program delayvalue for the battery 38 at full capacity may be smaller relative to theprogram delay value for battery 38 at half capacity. For illustrativepurposes, the program delay value for the battery at full capacity maybe 1 μs, whereas the program delay value for the battery at halfcapacity is defined at 100 μs. Continuing with the illustrative example,the program delay value for the battery at a quarter capacity may bedefined at 200 μs.

In other words, the program delay value will have a progressivelyincreasing duration as the remaining capacity or remaining energy storedby the battery 38 decreases. The increase in the duration defined by theprogram delay value accounts for the inventors' recognition—that thebattery and associated energy supply components require an increasingamount of time to recover operational power for supplying currentdemands as the battery capacity declines.

If the PROGRAM command is successfully executed, the state machinereturns to state SO. Techniques for determining whether the PROGRAMcommand has been successfully executed are known and will not bediscussed in detail herein. Suffice it to note that, any known errordetection and correction schemes may be implemented to verify theaccuracy of the data that is written to each segment, element and/orsector of the flash memory array.

The state machine is capable of “going through” all the states S0, S1,S2 and S3 (i.e. rotate in sequence through such states), as representedwith the full line, as a function of the memory array functions and/orof external events. In various embodiments, the working parameters ofthe state machine can be varied by software means.

FIG. 6A shows a plot of program time for a memory sector versus thecumulative current drain without implementing a program delay. Asdiscussed above, the program operation includes an erase that isfollowed by writing of data. In the program operation of FIG. 6A, theentire sector is written-to following an erase, without a program delay.As such, in the implementation of the state machine of FIG. 5, thesequence of states goes from S1 to S3 without the step of introducing adelay—i.e., no S2. This is akin to the conventional operation of thestate machines of conventional flash memory arrays.

FIG. 6B shows a plot of the program time for a memory sector versus thecumulative current drain with a program delay that is selected basedupon the capacity of the battery. By providing a delay between theprogram operations of the two or more memory elements of a sector, thebattery 38 is afforded sufficient time to recover operational power tosupport the peak power demands. In the implementation of the programdelay of FIG. 6B, the delay may cause the current drain during theprogram (writing) operation to be reduced to match the current drainduring the erase operation.

In comparison to the plot of FIG. 6A, the duration of completion of theprogram operation in FIG. 6B is longer. The legend in the plots of FIGS.6A and 6B (right-hand section) shows the amount of current drain toerase then program and the moving average current drain to erase andprogram full sector. Depending on the particular implementation, onebenefit of providing a delay between the programming of the memoryelements in a given sector is that the current drain associated witheach programming operation is decreased. In one experimental comparisonof the two implementations, the current drain associated with animplementation without a program delay was found to be about 800 pampswhereas the current drain associated with the implementation with aprogram delay was found to be about 180 pamps.

Accordingly, the techniques of the present disclosure facilitateminiaturization of devices, such as IMD 10, that are configured forimplantation in a patient. In accordance with aspects of the presentinvention, implementing a memory device with a programming delay enablesthe downsizing of the battery size and capacity of an implantablemedical device. The reduced form factor of the battery facilitates theminiaturization of the implantable medical devices. The memory devicesof the present disclosure may include non-transitory computer readablestorage media storing instructions that, when executed by one or moreprocessing circuits, cause the modules to perform various functionsattributed to IMD 10. The storage media may include anycomputer-readable storage media with the sole exception being atransitory, propagating signal.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment. It should also be understood that various changes can bemade in the function and arrangement of elements without departing fromthe scope of the disclosure as set forth in the appended claims and thelegal equivalents thereof.

What is claimed is:
 1. A method of programming a memory device of animplantable medical device, comprising: computing a parameter of abattery of the implantable medical device; erasing a memory sector ofthe memory device; performing an iterative programming of a plurality ofelements in the memory sector; applying a programming delay prior toprogramming at least one of the plurality of elements, wherein theprogramming delay is a function of the computed parameter.
 2. The methodof claim 1, wherein a value of the programming delay is defined as afunction of the parameter of the battery.
 3. The method of claim 1,wherein the computed parameter is a remaining amount of energy stored bythe battery.
 4. The method of claim 1, wherein the programming delay isduration between a programming of data to a first memory element and aprogramming of data to a second memory element.
 5. The method of claim1, further comprising receiving a memory address designator indicatingthe memory sector to be programmed.